A company I used to work for used to use the Versant OODBMS (object-oriented dbms). It was truly interesting. Long story short, you stored your objects in the

A company I used to work for used to use the Versant OODBMS (object-oriented dbms). It was truly interesting. Long story short, you stored your objects in the database, along with other objects. No object-relational mismatch. Queries meant taking a subset of a graph (of objects). It was fast and performant, and fairly solid. It's essentially the result of asking "what if database development had taken a different turn at some point?". Of the owning company would release it under some kind of open source license (maybe open core, BSL or one of those new revenue-friendly licenses) it could probably get very very popular.